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30. System Management

This chapter describes facilities for controlling the system that underlies a process (including the operating system and hardware) and for getting information about it. Anyone can generally use the informational facilities, but usually only a properly privileged process can make changes.

To get information on parameters of the system that are built into the system, such as the maximum length of a filename, System Configuration Parameters.


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30.1 Host Identification

This section explains how to identify the particular system on which your program is running. First, let's review the various ways computer systems are named, which is a little complicated because of the history of the development of the Internet.

Every Unix system (also known as a host) has a host name, whether it's connected to a network or not. In its simplest form, as used before computer networks were an issue, it's just a word like `chicken'.

But any system attached to the Internet or any network like it conforms to a more rigorous naming convention as part of the Domain Name System (DNS). In DNS, every host name is composed of two parts:

  1. hostname
  2. domain name

You will note that "hostname" looks a lot like "host name", but is not the same thing, and that people often incorrectly refer to entire host names as "domain names."

In DNS, the full host name is properly called the FQDN (Fully Qualified Domain Name) and consists of the hostname, then a period, then the domain name. The domain name itself usually has multiple components separated by periods. So for example, a system's hostname may be `chicken' and its domain name might be `ai.mit.edu', so its FQDN (which is its host name) is `chicken.ai.mit.edu'.

Adding to the confusion, though, is that DNS is not the only name space in which a computer needs to be known. Another name space is the NIS (aka YP) name space. For NIS purposes, there is another domain name, which is called the NIS domain name or the YP domain name. It need not have anything to do with the DNS domain name.

Confusing things even more is the fact that in DNS, it is possible for multiple FQDNs to refer to the same system. However, there is always exactly one of them that is the true host name, and it is called the canonical FQDN.

In some contexts, the host name is called a "node name."

For more information on DNS host naming, see Host Names.

Prototypes for these functions appear in `unistd.h'.

The programs hostname, hostid, and domainname work by calling these functions.

Function: int gethostname (char *name, size_t size)

This function returns the host name of the system on which it is called, in the array name. The size argument specifies the size of this array, in bytes. Note that this is not the DNS hostname. If the system participates in DNS, this is the FQDN (see above).

The return value is 0 on success and -1 on failure. In the GNU C library, gethostname fails if size is not large enough; then you can try again with a larger array. The following errno error condition is defined for this function:

ENAMETOOLONG

The size argument is less than the size of the host name plus one.

On some systems, there is a symbol for the maximum possible host name length: MAXHOSTNAMELEN. It is defined in `sys/param.h'. But you can't count on this to exist, so it is cleaner to handle failure and try again.

gethostname stores the beginning of the host name in name even if the host name won't entirely fit. For some purposes, a truncated host name is good enough. If it is, you can ignore the error code.

Function: int sethostname (const char *name, size_t length)

The sethostname function sets the host name of the system that calls it to name, a string with length length. Only privileged processes are permitted to do this.

Usually sethostname gets called just once, at system boot time. Often, the program that calls it sets it to the value it finds in the file /etc/hostname.

Be sure to set the host name to the full host name, not just the DNS hostname (see above).

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM

This process cannot set the host name because it is not privileged.

Function: int getdomainnname (char *name, size_t length)

getdomainname returns the NIS (aka YP) domain name of the system on which it is called. Note that this is not the more popular DNS domain name. Get that with gethostname.

The specifics of this function are analogous to gethostname, above.

Function: int setdomainname (const char *name, size_t length)

getdomainname sets the NIS (aka YP) domain name of the system on which it is called. Note that this is not the more popular DNS domain name. Set that with sethostname.

The specifics of this function are analogous to sethostname, above.

Function: long int gethostid (void)

This function returns the "host ID" of the machine the program is running on. By convention, this is usually the primary Internet IP address of that machine, converted to a long int. However, on some systems it is a meaningless but unique number which is hard-coded for each machine.

This is not widely used. It arose in BSD 4.2, but was dropped in BSD 4.4. It is not required by POSIX.

The proper way to query the IP address is to use gethostbyname on the results of gethostname. For more information on IP addresses, See section Host Addresses.

Function: int sethostid (long int id)

The sethostid function sets the "host ID" of the host machine to id. Only privileged processes are permitted to do this. Usually it happens just once, at system boot time.

The proper way to establish the primary IP address of a system is to configure the IP address resolver to associate that IP address with the system's host name as returned by gethostname. For example, put a record for the system in `/etc/hosts'.

See gethostid above for more information on host ids.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM

This process cannot set the host name because it is not privileged.

ENOSYS

The operating system does not support setting the host ID. On some systems, the host ID is a meaningless but unique number hard-coded for each machine.


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30.2 Platform Type Identification

You can use the uname function to find out some information about the type of computer your program is running on. This function and the associated data type are declared in the header file `sys/utsname.h'.

As a bonus, uname also gives some information identifying the particular system your program is running on. This is the same information which you can get with functions targetted to this purpose described in Host Identification.

Data Type: struct utsname

The utsname structure is used to hold information returned by the uname function. It has the following members:

char sysname[]

This is the name of the operating system in use.

char release[]

This is the current release level of the operating system implementation.

char version[]

This is the current version level within the release of the operating system.

char machine[]

This is a description of the type of hardware that is in use.

Some systems provide a mechanism to interrogate the kernel directly for this information. On systems without such a mechanism, the GNU C library fills in this field based on the configuration name that was specified when building and installing the library.

GNU uses a three-part name to describe a system configuration; the three parts are cpu, manufacturer and system-type, and they are separated with dashes. Any possible combination of three names is potentially meaningful, but most such combinations are meaningless in practice and even the meaningful ones are not necessarily supported by any particular GNU program.

Since the value in machine is supposed to describe just the hardware, it consists of the first two parts of the configuration name: `cpu-manufacturer'. For example, it might be one of these:

"sparc-sun", "i386-anything", "m68k-hp", "m68k-sony", "m68k-sun", "mips-dec"

char nodename[]

This is the host name of this particular computer. In the GNU C library, the value is the same as that returned by gethostname; see Host Identification.

 gethostname() is implemented with a call to uname().

char domainname[]

This is the NIS or YP domain name. It is the same value returned by getdomainname; see Host Identification. This element is a relatively recent invention and use of it is not as portable as use of the rest of the structure.

Function: int uname (struct utsname *info)

The uname function fills in the structure pointed to by info with information about the operating system and host machine. A non-negative value indicates that the data was successfully stored.

-1 as the value indicates an error. The only error possible is EFAULT, which we normally don't mention as it is always a possibility.


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30.3 Controlling and Querying Mounts

All files are in filesystems, and before you can access any file, its filesystem must be mounted. Because of Unix's concept of Everything is a file, mounting of filesystems is central to doing almost anything. This section explains how to find out what filesystems are currently mounted and what filesystems are available for mounting, and how to change what is mounted.

The classic filesystem is the contents of a disk drive. The concept is considerably more abstract, though, and lots of things other than disk drives can be mounted.

Some block devices don't correspond to traditional devices like disk drives. For example, a loop device is a block device whose driver uses a regular file in another filesystem as its medium. So if that regular file contains appropriate data for a filesystem, you can by mounting the loop device essentially mount a regular file.

Some filesystems aren't based on a device of any kind. The "proc" filesystem, for example, contains files whose data is made up by the filesystem driver on the fly whenever you ask for it. And when you write to it, the data you write causes changes in the system. No data gets stored.


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30.3.1 Mount Information

For some programs it is desirable and necessary to access information about whether a certain filesystem is mounted and, if it is, where, or simply to get lists of all the available filesystems. The GNU libc provides some functions to retrieve this information portably.

Traditionally Unix systems have a file named `/etc/fstab' which describes all possibly mounted filesystems. The mount program uses this file to mount at startup time of the system all the necessary filesystems. The information about all the filesystems actually mounted is normally kept in a file named either `/var/run/mtab' or `/etc/mtab'. Both files share the same syntax and it is crucial that this syntax is followed all the time. Therefore it is best to never directly write the files. The functions described in this section can do this and they also provide the functionality to convert the external textual representation to the internal representation.

Note that the `fstab' and `mtab' files are maintained on a system by convention. It is possible for the files not to exist or not to be consistent with what is really mounted or available to mount, if the system's administration policy allows it. But programs that mount and unmount filesystems typically maintain and use these files as described herein.

The filenames given above should never be used directly. The portable way to handle these file is to use the macro _PATH_FSTAB, defined in `fstab.h', or _PATH_MNTTAB, defined in `mntent.h' and `paths.h', for `fstab'; and the macro _PATH_MOUNTED, also defined in `mntent.h' and `paths.h', for `mtab'. There are also two alternate macro names FSTAB, MNTTAB, and MOUNTED defined but these names are deprecated and kept only for backward compatibility. The names _PATH_MNTTAB and _PATH_MOUNTED should always be used.


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30.3.1.1 The `fstab' file

The internal representation for entries of the file is struct fstab, defined in `fstab.h'.

Data Type: struct fstab

This structure is used with the getfsent, getfsspec, and getfsfile functions.

char *fs_spec

This element describes the device from which the filesystem is mounted. Normally this is the name of a special device, such as a hard disk partition, but it could also be a more or less generic string. For NFS it would be a hostname and directory name combination.

Even though the element is not declared const it shouldn't be modified. The missing const has historic reasons, since this function predates ISO C. The same is true for the other string elements of this structure.

char *fs_file

This describes the mount point on the local system. I.e., accessing any file in this filesystem has implicitly or explicitly this string as a prefix.

char *fs_vfstype

This is the type of the filesystem. Depending on what the underlying kernel understands it can be any string.

char *fs_mntops

This is a string containing options passed to the kernel with the mount call. Again, this can be almost anything. There can be more than one option, separated from the others by a comma. Each option consists of a name and an optional value part, introduced by an = character.

If the value of this element must be processed it should ideally be done using the getsubopt function; see Parsing of Suboptions.

const char *fs_type

This name is poorly chosen. This element points to a string (possibly in the fs_mntops string) which describes the modes with which the filesystem is mounted. `fstab' defines five macros to describe the possible values:

FSTAB_RW

The filesystems gets mounted with read and write enabled.

FSTAB_RQ

The filesystems gets mounted with read and write enabled. Write access is restricted by quotas.

FSTAB_RO

The filesystem gets mounted read-only.

FSTAB_SW

This is not a real filesystem, it is a swap device.

FSTAB_XX

This entry from the `fstab' file is totally ignored.

Testing for equality with these value must happen using strcmp since these are all strings. Comparing the pointer will probably always fail.

int fs_freq

This element describes the dump frequency in days.

int fs_passno

This element describes the pass number on parallel dumps. It is closely related to the dump utility used on Unix systems.

To read the entire content of the of the `fstab' file the GNU libc contains a set of three functions which are designed in the usual way.

Function: int setfsent (void)

This function makes sure that the internal read pointer for the `fstab' file is at the beginning of the file. This is done by either opening the file or resetting the read pointer.

Since the file handle is internal to the libc this function is not thread-safe.

This function returns a non-zero value if the operation was successful and the getfs* functions can be used to read the entries of the file.

Function: void endfsent (void)

This function makes sure that all resources acquired by a prior call to setfsent (explicitly or implicitly by calling getfsent) are freed.

Function: struct fstab * getfsent (void)

This function returns the next entry of the `fstab' file. If this is the first call to any of the functions handling `fstab' since program start or the last call of endfsent, the file will be opened.

The function returns a pointer to a variable of type struct fstab. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred getfsent returns a NULL pointer.

Function: struct fstab * getfsspec (const char *name)

This function returns the next entry of the `fstab' file which has a string equal to name pointed to by the fs_spec element. Since there is normally exactly one entry for each special device it makes no sense to call this function more than once for the same argument. If this is the first call to any of the functions handling `fstab' since program start or the last call of endfsent, the file will be opened.

The function returns a pointer to a variable of type struct fstab. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred getfsent returns a NULL pointer.

Function: struct fstab * getfsfile (const char *name)

This function returns the next entry of the `fstab' file which has a string equal to name pointed to by the fs_file element. Since there is normally exactly one entry for each mount point it makes no sense to call this function more than once for the same argument. If this is the first call to any of the functions handling `fstab' since program start or the last call of endfsent, the file will be opened.

The function returns a pointer to a variable of type struct fstab. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred getfsent returns a NULL pointer.


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30.3.1.2 The `mtab' file

The following functions and data structure access the `mtab' file.

Data Type: struct mntent

This structure is used with the getmntent, getmntent_t, addmntent, and hasmntopt functions.

char *mnt_fsname

This element contains a pointer to a string describing the name of the special device from which the filesystem is mounted. It corresponds to the fs_spec element in struct fstab.

char *mnt_dir

This element points to a string describing the mount point of the filesystem. It corresponds to the fs_file element in struct fstab.

char *mnt_type

mnt_type describes the filesystem type and is therefore equivalent to fs_vfstype in struct fstab. `mntent.h' defines a few symbolic names for some of the values this string can have. But since the kernel can support arbitrary filesystems it does not make much sense to give them symbolic names. If one knows the symbol name one also knows the filesystem name. Nevertheless here follows the list of the symbols provided in `mntent.h'.

MNTTYPE_IGNORE

This symbol expands to "ignore". The value is sometime used in `fstab' files to make sure entries are not used without removing them.

MNTTYPE_NFS

Expands to "nfs". Using this macro sometimes could make sense since it names the default NFS implementation, in case both version 2 and 3 are supported.

MNTTYPE_SWAP

This symbol expands to "swap". It names the special `fstab' entry which names one of the possibly multiple swap partitions.

char *mnt_opts

The element contains a string describing the options used while mounting the filesystem. As for the equivalent element fs_mntops of struct fstab it is best to use the function getsubopt (see section Parsing of Suboptions) to access the parts of this string.

The `mntent.h' file defines a number of macros with string values which correspond to some of the options understood by the kernel. There might be many more options which are possible so it doesn't make much sense to rely on these macros but to be consistent here is the list:

MNTOPT_DEFAULTS

Expands to "defaults". This option should be used alone since it indicates all values for the customizable values are chosen to be the default.

MNTOPT_RO

Expands to "ro". See the FSTAB_RO value, it means the filesystem is mounted read-only.

MNTOPT_RW

Expand to "rw". See the FSTAB_RW value, it means the filesystem is mounted with read and write permissions.

MNTOPT_SUID

Expands to "suid". This means that the SUID bit (see section How an Application Can Change Persona) is respected when a program from the filesystem is started.

MNTOPT_NOSUID

Expands to "nosuid". This is the opposite of MNTOPT_SUID, the SUID bit for all files from the filesystem is ignored.

MNTOPT_NOAUTO

Expands to "noauto". At startup time the mount program will ignore this entry if it is started with the -a option to mount all filesystems mentioned in the `fstab' file.

As for the FSTAB_* entries introduced above it is important to use strcmp to check for equality.

mnt_freq

This elements corresponds to fs_freq and also specifies the frequency in days in which dumps are made.

mnt_passno

This element is equivalent to fs_passno with the same meaning which is uninteresting for all programs beside dump.

For accessing the `mtab' file there is again a set of three functions to access all entries in a row. Unlike the functions to handle `fstab' these functions do not access a fixed file and there is even a thread safe variant of the get function. Beside this the GNU libc contains functions to alter the file and test for specific options.

Function: FILE * setmntent (const char *file, const char *mode)

The setmntent function prepares the file named FILE which must be in the format of a `fstab' and `mtab' file for the upcoming processing through the other functions of the family. The mode parameter can be chosen in the way the opentype parameter for fopen (see section Opening Streams) can be chosen. If the file is opened for writing the file is also allowed to be empty.

If the file was successfully opened setmntent returns a file descriptor for future use. Otherwise the return value is NULL and errno is set accordingly.

Function: int endmntent (FILE *stream)

This function takes for the stream parameter a file handle which previously was returned from the setmntent call. endmntent closes the stream and frees all resources.

The return value is 1 unless an error occurred in which case it is 0.

Function: struct mntent * getmntent (FILE *stream)

The getmntent function takes as the parameter a file handle previously returned by successful call to setmntent. It returns a pointer to a static variable of type struct mntent which is filled with the information from the next entry from the file currently read.

The file format used prescribes the use of spaces or tab characters to separate the fields. This makes it harder to use name containing one of these characters (e.g., mount points using spaces). Therefore these characters are encoded in the files and the getmntent function takes care of the decoding while reading the entries back in. '\040' is used to encode a space character, '\011' to encode a tab character, '\012' to encode a newline character, and '\\' to encode a backslash.

If there was an error or the end of the file is reached the return value is NULL.

This function is not thread-safe since all calls to this function return a pointer to the same static variable. getmntent_r should be used in situations where multiple threads access the file.

Function: struct mntent * getmntent_r (FILE *stream, struct mentent *result, char *buffer, int bufsize)

The getmntent_r function is the reentrant variant of getmntent. It also returns the next entry from the file and returns a pointer. The actual variable the values are stored in is not static, though. Instead the function stores the values in the variable pointed to by the result parameter. Additional information (e.g., the strings pointed to by the elements of the result) are kept in the buffer of size bufsize pointed to by buffer.

Escaped characters (space, tab, backslash) are converted back in the same way as it happens for getmentent.

The function returns a NULL pointer in error cases. Errors could be:

Function: int addmntent (FILE *stream, const struct mntent *mnt)

The addmntent function allows adding a new entry to the file previously opened with setmntent. The new entries are always appended. I.e., even if the position of the file descriptor is not at the end of the file this function does not overwrite an existing entry following the current position.

The implication of this is that to remove an entry from a file one has to create a new file while leaving out the entry to be removed and after closing the file remove the old one and rename the new file to the chosen name.

This function takes care of spaces and tab characters in the names to be written to the file. It converts them and the backslash character into the format describe in the getmntent description above.

This function returns 0 in case the operation was successful. Otherwise the return value is 1 and errno is set appropriately.

Function: char * hasmntopt (const struct mntent *mnt, const char *opt)

This function can be used to check whether the string pointed to by the mnt_opts element of the variable pointed to by mnt contains the option opt. If this is true a pointer to the beginning of the option in the mnt_opts element is returned. If no such option exists the function returns NULL.

This function is useful to test whether a specific option is present but when all options have to be processed one is better off with using the getsubopt function to iterate over all options in the string.


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30.3.1.3 Other (Non-libc) Sources of Mount Information

On a system with a Linux kernel and the proc filesystem, you can get information on currently mounted filesystems from the file `mounts' in the proc filesystem. Its format is similar to that of the `mtab' file, but represents what is truly mounted without relying on facilities outside the kernel to keep `mtab' up to date.


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30.3.2 Mount, Unmount, Remount

This section describes the functions for mounting, unmounting, and remounting filesystems.

Only the superuser can mount, unmount, or remount a filesystem.

These functions do not access the `fstab' and `mtab' files. You should maintain and use these separately. See section Mount Information.

The symbols in this section are declared in `sys/mount.h'.

Function: int mount (const char *special_file, const char *dir, const char *fstype, unsigned long int options, const void *data)

mount mounts or remounts a filesystem. The two operations are quite different and are merged rather unnaturally into this one function. The MS_REMOUNT option, explained below, determines whether mount mounts or remounts.

For a mount, the filesystem on the block device represented by the device special file named special_file gets mounted over the mount point dir. This means that the directory dir (along with any files in it) is no longer visible; in its place (and still with the name dir) is the root directory of the filesystem on the device.

As an exception, if the filesystem type (see below) is one which is not based on a device (e.g. "proc"), mount instantiates a filesystem and mounts it over dir and ignores special_file.

For a remount, dir specifies the mount point where the filesystem to be remounted is (and remains) mounted and special_file is ignored. Remounting a filesystem means changing the options that control operations on the filesystem while it is mounted. It does not mean unmounting and mounting again.

For a mount, you must identify the type of the filesystem as fstype. This type tells the kernel how to access the filesystem and can be thought of as the name of a filesystem driver. The acceptable values are system dependent. On a system with a Linux kernel and the proc filesystem, the list of possible values is in the file `filesystems' in the proc filesystem (e.g. type cat /proc/filesystems to see the list). With a Linux kernel, the types of filesystems that mount can mount, and their type names, depends on what filesystem drivers are configured into the kernel or loaded as loadable kernel modules. An example of a common value for fstype is ext2.

For a remount, mount ignores fstype.

options specifies a variety of options that apply until the filesystem is unmounted or remounted. The precise meaning of an option depends on the filesystem and with some filesystems, an option may have no effect at all. Furthermore, for some filesystems, some of these options (but never MS_RDONLY) can be overridden for individual file accesses via ioctl.

options is a bit string with bit fields defined using the following mask and masked value macros:

MS_MGC_MASK

This multibit field contains a magic number. If it does not have the value MS_MGC_VAL, mount assumes all the following bits are zero and the data argument is a null string, regardless of their actual values.

MS_REMOUNT

This bit on means to remount the filesystem. Off means to mount it.

MS_RDONLY

This bit on specifies that no writing to the filesystem shall be allowed while it is mounted. This cannot be overridden by ioctl. This option is available on nearly all filesystems.

S_IMMUTABLE

This bit on specifies that no writing to the files in the filesystem shall be allowed while it is mounted. This can be overridden for a particular file access by a properly privileged call to ioctl. This option is a relatively new invention and is not available on many filesystems.

S_APPEND

This bit on specifies that the only file writing that shall be allowed while the filesystem is mounted is appending. Some filesystems allow this to be overridden for a particular process by a properly privileged call to ioctl. This is a relatively new invention and is not available on many filesystems.

MS_NOSUID

This bit on specifies that Setuid and Setgid permissions on files in the filesystem shall be ignored while it is mounted.

MS_NOEXEC

This bit on specifies that no files in the filesystem shall be executed while the filesystem is mounted.

MS_NODEV

This bit on specifies that no device special files in the filesystem shall be accessible while the filesystem is mounted.

MS_SYNCHRONOUS

This bit on specifies that all writes to the filesystem while it is mounted shall be synchronous; i.e., data shall be synced before each write completes rather than held in the buffer cache.

MS_MANDLOCK

This bit on specifies that mandatory locks on files shall be permitted while the filesystem is mounted.

MS_NOATIME

This bit on specifies that access times of files shall not be updated when the files are accessed while the filesystem is mounted.

MS_NODIRATIME

This bit on specifies that access times of directories shall not be updated when the directories are accessed while the filesystem in mounted.

Any bits not covered by the above masks should be set off; otherwise, results are undefined.

The meaning of data depends on the filesystem type and is controlled entirely by the filesystem driver in the kernel.

Example:

 
#include <sys/mount.h>

mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, "");

mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, "");

   

Appropriate arguments for mount are conventionally recorded in the `fstab' table. See section Mount Information.

The return value is zero if the mount or remount is successful. Otherwise, it is -1 and errno is set appropriately. The values of errno are filesystem dependent, but here is a general list:

EPERM

The process is not superuser.

ENODEV

The file system type fstype is not known to the kernel.

ENOTBLK

The file dev is not a block device special file.

EBUSY
  • The device is already mounted.
  • The mount point is busy. (E.g. it is some process' working directory or has a filesystem mounted on it already).
  • The request is to remount read-only, but there are files open for write.
EINVAL
  • A remount was attempted, but there is no filesystem mounted over the specified mount point.
  • The supposed filesystem has an invalid superblock.
EACCES
  • The filesystem is inherently read-only (possibly due to a switch on the device) and the process attempted to mount it read/write (by setting the MS_RDONLY bit off).
  • special_file or dir is not accessible due to file permissions.
  • special_file is not accessible because it is in a filesystem that is mounted with the MS_NODEV option.
EM_FILE

The table of dummy devices is full. mount needs to create a dummy device (aka "unnamed" device) if the filesystem being mounted is not one that uses a device.

Function: int umount2 (const char *file, int flags)

umount2 unmounts a filesystem.

You can identify the filesystem to unmount either by the device special file that contains the filesystem or by the mount point. The effect is the same. Specify either as the string file.

flags contains the one-bit field identified by the following mask macro:

MNT_FORCE

This bit on means to force the unmounting even if the filesystem is busy, by making it unbusy first. If the bit is off and the filesystem is busy, umount2 fails with errno = EBUSY. Depending on the filesystem, this may override all, some, or no busy conditions.

All other bits in flags should be set to zero; otherwise, the result is undefined.

Example:

 
#include <sys/mount.h>

umount2("/mnt", MNT_FORCE);

umount2("/dev/hdd1", 0);

   

After the filesystem is unmounted, the directory that was the mount point is visible, as are any files in it.

As part of unmounting, umount2 syncs the filesystem.

If the unmounting is successful, the return value is zero. Otherwise, it is -1 and errno is set accordingly:

EPERM

The process is not superuser.

EBUSY

The filesystem cannot be unmounted because it is busy. E.g. it contains a directory that is some process's working directory or a file that some process has open. With some filesystems in some cases, you can avoid this failure with the MNT_FORCE option.

EINVAL

file validly refers to a file, but that file is neither a mount point nor a device special file of a currently mounted filesystem.

This function is not available on all systems.

Function: int umount (const char *file)

umount does the same thing as umount2 with flags set to zeroes. It is more widely available than umount2 but since it lacks the possibility to forcefully unmount a filesystem is deprecated when umount2 is also available.


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30.4 System Parameters

This section describes the sysctl function, which gets and sets a variety of system parameters.

The symbols used in this section are declared in the file `sysctl.h'.

Function: int sysctl (int *names, int nlen, void *oldval, size_t *oldlenp, void *newval, size_t newlen)

sysctl gets or sets a specified system parameter. There are so many of these parameters that it is not practical to list them all here, but here are some examples:

The set of available parameters depends on the kernel configuration and can change while the system is running, particularly when you load and unload loadable kernel modules.

The system parameters with which syslog is concerned are arranged in a hierarchical structure like a hierarchical filesystem. To identify a particular parameter, you specify a path through the structure in a way analogous to specifying the pathname of a file. Each component of the path is specified by an integer and each of these integers has a macro defined for it by `sysctl.h'. names is the path, in the form of an array of integers. Each component of the path is one element of the array, in order. nlen is the number of components in the path.

For example, the first component of the path for all the paging parameters is the value CTL_VM. For the free page thresholds, the second component of the path is VM_FREEPG. So to get the free page threshold values, make names an array containing the two elements CTL_VM and VM_FREEPG and make nlen = 2.

The format of the value of a parameter depends on the parameter. Sometimes it is an integer; sometimes it is an ASCII string; sometimes it is an elaborate structure. In the case of the free page thresholds used in the example above, the parameter value is a structure containing several integers.

In any case, you identify a place to return the parameter's value with oldval and specify the amount of storage available at that location as *oldlenp. *oldlenp does double duty because it is also the output location that contains the actual length of the returned value.

If you don't want the parameter value returned, specify a null pointer for oldval.

To set the parameter, specify the address and length of the new value as newval and newlen. If you don't want to set the parameter, specify a null pointer as newval.

If you get and set a parameter in the same sysctl call, the value returned is the value of the parameter before it was set.

Each system parameter has a set of permissions similar to the permissions for a file (including the permissions on directories in its path) that determine whether you may get or set it. For the purposes of these permissions, every parameter is considered to be owned by the superuser and Group 0 so processes with that effective uid or gid may have more access to system parameters. Unlike with files, the superuser does not invariably have full permission to all system parameters, because some of them are designed not to be changed ever.

sysctl returns a zero return value if it succeeds. Otherwise, it returns -1 and sets errno appropriately. Besides the failures that apply to all system calls, the following are the errno codes for all possible failures:

EPERM

The process is not permitted to access one of the components of the path of the system parameter or is not permitted to access the system parameter itself in the way (read or write) that it requested.

ENOTDIR

There is no system parameter corresponding to name.

EFAULT

oldval is not null, which means the process wanted to read the parameter, but *oldlenp is zero, so there is no place to return it.

EINVAL
  • The process attempted to set a system parameter to a value that is not valid for that parameter.
  • The space provided for the return of the system parameter is not the right size for that parameter.
ENOMEM

This value may be returned instead of the more correct EINVAL in some cases where the space provided for the return of the system parameter is too small.

If you have a Linux kernel with the proc filesystem, you can get and set most of the same parameters by reading and writing to files in the sys directory of the proc filesystem. In the sys directory, the directory structure represents the hierarchical structure of the parameters. E.g. you can display the free page thresholds with

 
cat /proc/sys/vm/freepages
   

Some more traditional and more widely available, though less general, GNU C library functions for getting and setting some of the same system parameters are:


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